1. Field of the Invention
This invention relates generally to analog-to-digital converters and more particularly, it relates to an analog-to-digital converter system with dither control which allows a user to selectively add dither to the converter system output codes dependent upon the particular application.
2. Description of the Prior Art
As is generally well known in the field of digital processing applications, an analog-to-digital converter (ADC) is used to perform the conversion of an analog signal to a digital signal. In recent years, there has been a significant increase in the use of analog-to-digital converters in the audio applications as well as in the instrumentation applications. Typically, analog-to-digital converters include circuitry for receiving an analog input signal and for generating a digital output signal which is proportional to the analog input signal.
One type of analog-to-digital converter that has been used quite frequently in the past few years is one that uses delta-sigma modulation in which an analog voltage is applied to the input of a delta-sigma modulator and its output thereof is filtered so as to remove modulator quantization noise. The delta-sigma A/D converter is basically formed of an analog modulator and a digital filter. The analog modulator oversamples the analog input signal and produces a low-resolution digital output signal, which is then processed by the digital filter to remove the modulator quantization noise. The output of the digital filter is a digital word which has its own inherent quantization noise.
Accordingly, there are known prior art techniques for reducing the effects of quantization error, whether it is produced by an analog modulator or it is the result of output word quantization, by adding a small amount of noise, referred to also as dither, to the converter system. For example, there is described in the aforementioned application, U.S. Ser. No. 10/346,532, a method and apparatus for automatically converting a word length of sample data being transmitted over a serial link. Based upon the desired word length, the sample data is either truncated or padded and an appropriate amount of dither is added to the sample words to reduce the distortion and quantization artifacts introduced by the word length conversion. This U.S. application Ser. No. 10/346,532 is assigned to the same assignee as the present invention and is hereby incorporated by reference in its entirety.
In addition, there is disclosed in U.S. Pat. No. 6,356,872 to Ka Yin Leung et al. a digital audio system for storing digital audio data on a digital audio media and retrieving the stored digital audio data in a playback operation which includes a noise shaping device for adding dither to an input signal prior to truncation. Further, for a discussion of the differences between different types of dithered quantization schemes and the various dither signals utilized in practical applications, reference is made to the publication by S. P. Lipschitz, R. A. Wannmaker and J. Vanderkooy, “Quantization and Dither: A Theoretical Survey”, J. Audio Eng. Soc., Vol. 40, No. 5, pp. 355–374, May 1992 (hereafter “the Lipschitz et al., paper”).
In the prior art discussed above, there are described ways of how the error effects of quantization can be removed by adding the proper amount of dither, i.e., quantization error dithered by noise. Dithering has particular applications such as in the case of averaging output conversion words so as to improve resolution or performing digital signal processing or sampling of very small AC signals since the quantization error would introduce effects which could be detrimental to the desired results. The use of dither with proper statistical properties can render the total error to be equivalent to a steady white noise (See p. 356 of the Lipschitz et al., paper). However, it is not always desirable to have the quantization error at the converter output be dithered.
Specifically, in instrumentation applications such as measuring a “dc” signal with only one conversion being performed and where the thermal noise of the analog-to-digital converter is only a fraction of a least significant bit (LSB), it would then be desirable to have the converter output codes dominated by quantization error instead of being properly dithered since it would yield the highest resolution for the measurement. For example, a theoretical 16-bit A/D converter is assumed to have thermal noise equal to the value of LSB/20. When such an ADC is used to perform a measurement on a dc signal and where only a single conversion is being performed, the maximum output code toggle that can occur would be two (2) codes. In this case, the ADC can produce up to 32,767 “noise-free” codes in its transfer function.
In view of this, it would be desirable to provide an analog-to-digital converter system with dither control which allows a user to selectively add dither to the converter output codes dependent upon the particular application. It would also be expedient that the analog-to-digital converter system include a pin input or a register control bit which is readily accessible by the user for controlling the converter system output to be dominated by quantization error or to be properly dithered.